The invention relates to a structural element for use as a core layer in a sandwich composite element, a sandwich composite element, particularly for producing wind blades for wind power installations and/or for uses in the maritime sector (particularly for the production of boat hulls and boat decks), in the rail-transport sector (train fronts, roofs, floors, wall elements of railway carriages), in mass transport by road (bus roofs, floors, fronts), for structural uses in the building sector (e.g. roofs) and also a method for producing a structural element of this type.
A generic structural element in described in EP 1 536 944 B2, which is suitable and intended for use as a core layer in a sandwich composite element for producing wind blades in wind power installations. The known structural element distinguishes itself by means of a plurality of body segments made from polyethylene terephthalate (PET) that are arranged next to one another in one plane and connected to one another, the body segments being welded at the abutting side faces thereof, forming flat weld seams which, in a plan view, interconnect on a face side of the structural element, the flat weld seams forming a low-pore or pore-free plastic intermediate layer made from fused plastic of the body segments in the form of a reticulated web structure, which acts in a reinforcing manner.
The known planiform structural element is obtained by dividing a foam block, comprising the multiplicity of mutually welded body segments, perpendicularly to the planar extent of the intersecting weld seams by sawing the same into a multiplicity of plate-shaped structural elements. The thus-obtained structural element is processed to form a sandwich composite element, in that the face sides formed by sawing from a foam block are each bonded to a cover layer (e.g. aluminium sheet) using adhesive resin or are directly laminated to a fibre-reinforced resin cover layer without a further adhesive layer. The known structural elements have proven themselves as core layers in such sandwich composite elements. However, there are efforts to reduce the resin absorption of the structural element, particularly in the case of equal adhesive force of the cover layer, in order to thus obtain a sandwich composite element with a lower overall thickness and thus, for the same volume, with a lower weight for the same mechanical loadability. In the case of the laminating process in particular, in which the laminating resin is sucked into the layer arrangement with the aid of a vacuum, a reduced resin absorption is particularly interesting, as the laminating resin has the tendency, because of the vacuum loading, to fill all reachable cavities, and thus makes up a comparatively large proportion of the total weight.
It is suggested in WO 2005/047377 A1, which is not concerned with PET foams, to design the foam to have finer pores as a solution for a similar problem. However, this ultimately leads to unsatisfactory adhesion or adhesive action between a structural element and a cover layer.
A further approach for a solution, according to which the foamed material is condensed by applying pressure and temperature, is described in WO 2004/007600 A1, which is likewise not concerned with PET foams. However, this leads to foams with comparatively high density and makes the production method uneconomical, owing to the additionally required work step.
US 2005/0060895 A1 is concerned with the production of surfboards from a plurality of foamed structural elements made from polystyrene, the structural elements being shaped and welded to one another in a common method step. It is known from the published document to use hot-wire cutting for shaping the mutually welded structural elements.
U.S. Pat. No. 6,213,540 A1 describes a method for producing energy absorption products, in which a foam block is run through using a net made up of hot wires, in order to thus generate the weld seams within the block, which strengthen the structure. This published document is also not concerned with reducing the resin absorption.
A method for producing a structural element for use as a core layer in a sandwich composite element is described in GB 2474431 A, plastic layers initially being extruded over one another, and the respectively lower layer being heated by means of an infra-red heater before the subsequent layer is extruded on, so that the layers integrally connect to one another. The layer arrangement is then divided by means of hot-wire cutting into large blocks, which are then in turn divided into plate-shaped structural elements by means of hot-wire cutting. The published document does not consider the problem of reducing the resin absorption of the structural element during the further processing thereof to form a sandwich composite element.